This disclosure relates generally to exhaust gas sensors, and specifically to reduction of inconsistencies in break-in performance in exhaust oxygen sensors.
Oxygen sensors are used in a variety of applications that require qualitative and quantitative analysis of gases. For example, oxygen sensors have been used for many years in automotive vehicles to sense the presence of oxygen in exhaust gases, such as when an exhaust gas content switches from rich to lean or lean to rich. In automotive applications, the direct relationship between oxygen concentration in the exhaust gas and the air-to-fuel ratio of the fuel mixture supplied to the engine allows the oxygen sensor to provide oxygen concentration measurements for determination of optimum combustion conditions, maximization of fuel economy, and the management of exhaust emissions.
A conventional stoichiometric oxygen sensor typically consists of an ionically conductive solid electrolyte material, a porous platinum electrode which is exposed to the exhaust gases, and a porous electrode on the sensor""s interior surface exposed to a known oxygen partial pressure. Sensors typically used in automotive applications use a yttria-stabilized, zirconia-based electrochemical galvanic cell operating in potentiometric mode to detect the relative amounts of oxygen present in an automobile engine""s exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia electrolyte, according to the Nernst equation:   E  =            (              RT                  4          ⁢          F                    )        ⁢          ln      ⁡              (                              P                          O              2                        ref                                P                          O              2                                      )            
where:
E=electromotive force
R=universal gas constant
F=Faraday constant
T=absolute temperature of the gas
PO2ref=oxygen partial pressure of the reference gas
PO2=oxygen partial pressure of the exhaust gas
Due to the large difference in oxygen partial pressures between fuel rich and fuel lean exhaust conditions, the electromotive force changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric oxygen sensors indicate qualitatively whether the engine is operating fuel rich or fuel lean, without quantifying the actual air to fuel ratio of the exhaust mixture.
When first put into use, exhaust oxygen sensors exhibit a xe2x80x9cgreenxe2x80x9d effect, which produces inconsistent performance during the initial use of the sensor. Engine calibration must typically account for the green effect, which makes calibration more difficult. After several hours of use, the green effect disappears, and more reliable sensor performance is seen.
To reduce the green effect, conventional oxygen sensors incorporate various elements into the ink used to form the electrodes. Sodium, magnesium, and potassium, in particular, have been incorporated into ink prior to electrode formation in an attempt to ameliorate the green effect. This approach, however, can incorporate excessive amounts of the elements in the finished sensor element, which causes a degradation in the performance of the sensor.
What is needed in the art is a gas sensor with a reduced green effect.
The above-described and other disadvantages of the prior art are overcome by the sensor element described herein. The exhaust gas sensor element comprises an electrolyte body having a first surface and a second surface. Disposed in intimate contact with the first surface is a first electrode, while a second electrode is disposed in intimate contact with the second surface. The second electrode comprises lead oxide in an amount of about 0.1 to about 8 mg/cm2.
The method for making the gas sensor element comprises forming an electrolyte body and forming an electrode ink comprising a first catalyst. The electrode ink is applied to a first surface and a second surface of the electrolyte body. The body is sintered to form a catalyst layer. Lead oxide is applied to the catalyst layer in an amount of about 0.1 to about 8 mg/cm2. A second catalyst is also applied to said catalyst layer, and the layer is sintering to form a first electrode and a second electrode.
The method for depositing lead oxide on a gas sensor element, comprises applying a lead oxide containing glass to a substrate. The gas sensor element is placed in a closed container with the substrate and the element is heated causing lead oxide to be liberated from the substrate in vapor form and adsorbed by the gas sensor element. The resulting sensor has a first electrode and a second electrode comprising lead oxide in an amount of about 0.1 to about 8 mg/cm2.
Finally, the gas sensor comprises a middle shell, with a lower shell and an upper shell disposed in contact with the middle shell. The sensor element is disposed in contact with the middle shell, protruding into the lower shell and the upper shell. At least one electrical connector disposed in contact with a first electrode and a second electrode of the sensor element, such that electrical access is provided to the sensor element from an external circuit.